Winan,
It's been a very long time since I had to do anything other than standard 8.5 by 11 for portrait/landscape...but my 'guess' is that when you're designing the report, go to "printer setup", select "paper size", and pick the paper size that you need. (that's assuming the paper size you want is one of the ones on the list of available paper sizes). Then you can design the report according to that size.
Now, that's based on your specific default printer/printer family when you design the report. It you're designing for 8.5 by 4.5 for one printer, and your users will run the report for a different printer and still want to print 8.5 by 4.5, I don't know for sure if that will work.
Kevin